1. Field of the Invention
This invention relates to a noise suppressor for use with an automotive vehicle control system including a digital computer using clock pulses for eliminating radio interference problems.
2. Description of the Prior Art
As inexpensive and high performance digital computers become available with advancing digital electronics technology, it becomes technically and economically desirable to use a digital computer to control an automotive vehicle internal combustion engine and other automotive vehicle components.
FIG. 1 is a block diagram of a prior art control system using a digital computer and FIGS. 2 and 3 are schematic views showing the location of an automotive digital computer.
Referring to FIG. 1, a digital computer 10 includes control units 11 to 14, a central processing unit (CPU) 15, and a memory 16. The control units 11 to 14 are respectively connected through input/output interfaces 21 to 24 to signal terminals 31 to 34, in turn connected to signal lines 41 to 44, respectively. The digital computer 10 is powered through a power unit 25, connected to a power terminal 35 and thence through a power line 45 to an external power source such as an automotive vehicle battery. The digital computer 10, the input/output interfaces 21 to 24 and the power unit 25 are enclosed by a metal casing 50.
As shown in FIG. 2, the metal casing 50 is installed below an automotive vehicle instrument panel or below a seat for the driver, at a location where temperature, moisture and other environment conditions are not particularly adverse. A bundle 40, beneath the vehicle hood, includes signal lines 41 to 44 and the power line 45.
In such a digital computer, relatively high voltages (several volts) rectangular clock pulses are used as computer control synchronizing pulses having fundamental, harmonic and higher harmonic frequencies over a wide range of radio frequency bands, including AM, FM and TV bands; the pulses cause radio interference. Since the signals normally fed through the signal lines 41 to 44 from various sensors and to the fuel injection valve drive coils are low frequency signals, signal lines 41 to 44 are commonly made of polyvinyl chloride tubes covering electric wires; lines 41 to 44 are wired in a bundle as are other electric wires connected to a radio receiver and other communication devices. All of the lines and wires are located in the bundle without any consideration of the possibility of leakage of high frequency signals through the signal lines. If rectangular clock pulses leak through the signal lines 41 to 44 due to electromagnetic coupling between the CPU 15 and the input/output interfaces 21 to 24, the clock pulses are coupled directly or indirectly to the other electric wires, causing radio interference. Electromagnetic coupling of noises radiating from the bundle of the signal lines 40 to an automotive vehicle receiver system including antenna 61 having thin conductors embedded in the vehicle front glass windshield, radio receiver 62, and an electric wire 63 leading to the radio receiver 62 is schematically shown in FIG. 3 by arrows.
In such a digital computer contained control system, many difficulties have been found in attempting to prevent leakage of rectangular clock pulses through the signal lines.
A first difficulty is that a relatively large number of design processes are required to determine a suitable layout to completely, electromagnetically separate parts, such as the CPU 15, memory 16 and the like, that operates with rectangular clock pulses, from the other parts, such as the input/output interfaces 21 to 24 and the signal lines 41 to 44.
The electromagnetic coupling between the signal lines and the other electric wires may be avoided by: (1) using a shielding cable such as a concentric cable as the signal lines, (2) shielding the signal lines from the other electric wires, or (3) spacing the signal lines from the other electric wires and the antenna as far as possible. These three techniques are impossible in practice if a great number of signal lines are provided, the wiring is complex, or the electric wires terminate at many points within the vehicle compartment.
If the digital system is placed adjacent the communication devices and rectangular clock pulses leak to the input/output signal lines, the signal-to-noise (S/N) ratio of the communication devices is reduced due to electromagnetic coupling between the signal lines and the electric wires leading to the communication devices and antenna; the ratio is further reduced due to beat interference.